In multicellular organisms, homeostasis is maintained by balancing the rate of cell proliferation against the rate of cell death. Cell proliferation is influenced by numerous growth factors and the expression of proto-oncogenes, which typically encourage progression through the cell cycle. In contrast, numerous events, including the expression of tumor suppressor genes, can lead to an arrest of cellular proliferation.
In differentiated cells, a particular type of cell death called apoptosis occurs when an internal suicide program is activated. This program can be initiated by a variety of external signals as well as signals that are generated within the cell in response to, for example, genetic damage. For many years, the magnitude of apoptotic cell death was not appreciated because the dying cells are quickly eliminated by phagocytes, without an inflammatory response.
The mechanisms that mediate apoptosis have been intensively studied. These mechanisms involve the activation of endogenous proteases, loss of mitochondrial function, and structural changes such as disruption of the cytoskeleton, cell shrinkage, membrane blebbing, and nuclear condensation due to degradation of DNA. The various signals that trigger apoptosis are thought to bring about these events by converging on a common cell death pathway that is regulated by the expression of genes that are highly conserved from worms, such as C. elegans, to humans. In fact, invertebrate model systems have been invaluable tools in identifying and characterizing the genes that control apoptosis. Through the study of invertebrates and more evolved animals, numerous genes that are associated with cell death have been identified, but the way in which their products interact to execute the apoptotic program is poorly understood.
Caspases, a class of proteins central to the apoptotic program, are cysteine protease having specificity for aspartate at the substrate cleavage site. These proteases are primarily responsible for the degradation of cellular proteins that lead to the morphological changes seen in cells undergoing apoptosis. For example, one of the caspases identified in humans was previously known as the interleukin-1.beta. (IL-1.beta.) converting enzyme (ICE), a cysteine protease responsible for the processing of pro-IL-1.beta. to the active cytokine. Overexpression of ICE in Rat-1 fibroblasts induces apoptosis (Miura et al., Cell 75:653 [1993]).
Many caspases and proteins that interact with caspases possess domains of about 60 amino acids called a caspase recruitment domain (CARD). Hofmann et al. (TIBS 22:155 [1997]) and others have postulated that certain apoptotic proteins bind to each other via their CARDs and that different subtypes of CARDs may confer binding specificity, regulating the activity of various caspases, for example.
The functional significance of CARDs have been demonstrated in two recent publications. Duan et al. (Nature 385:86 [1997]) showed that deleting the CARD at the N-terminus of RAIDD, a newly identified protein involved in apoptosis, abolished the ability of RAIDD to bind to caspases. In addition, Li et al. (Cell 91:479 [1997]) showed that the N-terminal 97 amino acids of apoptotic protease activating factor-1 (Apaf-1) was sufficient to confer caspase-9-binding ability.